As the continuous development of engine theory and technology, exhaust gas recirculation (EGR) system has become an important part of a diesel engine. Exhaust gas emitted from a diesel engine generally contains a great amount of nitrogen oxides (NOx), which is a major source for air pollution. With the EGR system, a part of exhaust gas generated by the diesel engine is fed back to cylinders. Since the recirculated exhaust gas is inertial, it will delay the combustion process, decelerate the combustion speed somewhat, further slow down the pressure formation process in the combustion chamber, thereby effectively reducing the nitrogen oxides. Besides, increase of the exhaust gas recirculation ratio will reduce the overall exhaust gas flow, thereby reducing the total pollutants output volume in the exhaust gas emission.
In a diesel engine equipped with the EGR system, the matching relation between exhaust gas recirculation ratio (EGR rate) and fresh air entering into the engine during transient process is key to the transient process emission of the air system of the diesel engine. Therefore, many diesel engines employ a turbocharge system to accelerate the response of the air system during transient process. Besides, the turbocharge system may enhance the power performance of the diesel engine and improve combustion, and is one of important components in modern diesel engines. For example, a variable geometry turbocharger (VGT) is a common turbocharge system. The turbocharge system is essentially an air compression system, in which air intake volume increases in the diesel engine cylinders through compressing air. It is driven by the impact force from the exhaust gas emitted from the engine. The pressure is transmitted to an air compressor through devices such as a turbocharger rotary shaft, such that the newly intake air is effectively supercharged before entering into the cylinders.
In a diesel engine equipped with both the EGR and the turbocharge system, the coupling characteristic therebetween poses a challenge to the control of air system. In the diesel engine equipped with both the exhaust gas recirculation system EGR and the turbocharge system, for the EGR system, precise control of the EGR rate and intake air temperature is crucial to improve NOx emission and reduce its impact on particles, power, and cost-effectiveness. In such an engine, the flow of the exhaust gas input in an EGR cooler is controlled by an EGR valve. Both the inlet end of the EGR valve and the turbo outlet of the turbocharger receive the engine exhaust gas emitted from the exhaust pipes. It is appreciated that besides the opening variation of the EGR valve per se, the change of the supercharge pressure and exhaust back pressure caused by the turbocharge system will also cause an impact on the EGR flow. On the other hand, the opening variation of the EGR valve will also cause an impact on the inlet flow of the input supercharger. In other words, the exhaust gas recirculation system and the turbocharge system are two mutually dependent and mutually influencing systems, i.e., having a coupling characteristic. In particular, in an air system control of the diesel engine, the match between EGR rate and fresh air during transient process is key during the transient emission process.
The coupling characteristic of the exhaust gas recirculation system and the turbocharge system has always been a challenge for air system control of a diesel engine, and a multi-variable control strategy controlling both has also been a hot issue in studying air system control strategy of the diesel engine. Several known control strategies in prior art are briefly summarized below:
(1) an independent control strategy for exhaust gas recirculation system and the turbocharge system, i.e., with the supercharge pressure as control objective, driving the VGT valve by a PID (proportion-integration differentiation) control with transient feed-forward control strategy so that actual supercharge pressure reaches an objective value; with air flow as control objective, driving the EGR valve by PID control with transient feed-forward control strategy so that the actual air flow reaches an objective value.
(2) With intake air flow and supercharge pressure as control objectives, performing local linearization to average value model of air system, designing an optimal or robust controller based on the linear model, further extending to entire operation condition scope, thereby obtaining a non-linear control strategy: e.g., H infinity control, a controller design method based on Lyapunov stability theory, minimum quadratic model optimal state feedback control law, sliding mode controller, and the like.
(3) With intake air flow and supercharge pressure as control objectives, a controller design method based on a non-analytic model: e.g., fuzzy logic control method, control method according to neural network, etc.
(4) With intake air flow and supercharge pressure as control objectives, a method of employing model prediction control, i.e., a mathematic model of a controlled object being integrated in the controller, a future output of a multi-step system being predicted through the model, an objective function being built based on the offset between the predicted value and the objective value, and the objective function being minimized by iteratively evaluating the optimal value of current control variables.
(5) With air-fuel ratio and mass fraction of exhaust gas within the intake pipe as control objectives, adopting the air system de-ranking and de-coupling control strategy, i.e., the transmission function matrix of the air system is de-ranking in some cases; thus, the two control objectives have such a relationship that the original two-dimensional control strategy may be converted into a simpler one-dimensional control strategy.
The above mentioned major advantages of the independent PID control strategy (1) based on air flow and supercharge pressure lie in a simpler structure, the capability of implementing a good steady-state control effect, and less experimental workload for parameter calibration. The challenge of the independent closed-loop PID control lies in that the coupling characteristic of the system per se causes dissatisfactory control effect in its dynamic process, and smoking phenomenon likely appears during acceleration process. Another drawback of independently working closed-loop control lies in the limited EGR working scope. Because the EGR valve is only capable of working when the pressure before turbo is higher than the supercharge pressure, it is only applicable to medium-low load and medium-low rotational speed operation. Companies such as Nissan, Toyota, Cummmins do not employ air flow and supercharge pressure as the objective values during practical use, instead, they adopt a control strategy with the EGR rate instead of supercharge pressure as the objective value.
A common problem with the above mentioned methods is EGR flow estimation. Since EGR flow sensor is far away from meeting the requirement of actual use in terms of precision and reliability, the EGR flow is mainly obtained by estimation. However, exhaust pipes temperature and pressure, the EGR pipe throttling coefficient, and cooling efficiency and the like that place an impact on EGR flow all require a considerable amount of testing in order to obtain a satisfactory estimation result. Therefore, the control system according to this method requires enormous work for testing. Although the above mentioned control strategies are able to achieve a sound effect in a steady state control, their transient control effects are always unsatisfying since the exhaust gas recirculation system and the supercharge system simultaneously act on the intake pipe thus having a coupling characteristic, and those control strategies fail to design a transient control strategy for the coupling characteristic.
There is an apparent contradiction between precision requirement and concise requirement of the air system control strategy for control strategies (2)-(4) with intake air flow and supercharge pressure as control objectives. This contradiction is directly caused by the strong coupling and non-linear correlation between the exhaust gas recirculation system and the supercharge system. The independent closed-loop control strategies based on air flow and supercharge pressure, as well as its variations, cannot meet the requirements of steady state and transient performance. Various theoretical study outcomes are not adaptable for the requirements of an actual control system due to various factors such as complexity of control strategies, requirements of control hardware, and difficulties in parameter calibration, etc.
As far the control strategy (5) with the air-fuel ratio and the mass fraction of exhaust gas in the intake pipe as control objectives, due to lack of mature commercial sensor that directly measures the air-fuel ratio and the mass fraction of exhaust gas in the intake pipe during actual use, the feedback control with the parameters as control objectives cannot be realized. However, the air flow and supercharge pressure can be very easily measured by existing sensors. Therefore, a feedback strategy based on air flow and supercharge pressure may be built, and air-fuel ratio and exhaust gas mass fraction in the intake pipe both as intermediate variables may be obtained through an observer. However, the state observer would introduce time delay and error, which are disadvantageous to transient operation control.
In view of above, the control strategies for air system in a diesel engine in prior art can not well meet the performance requirements of steady state and transient operation at the same time during actual working of the diesel engine or the requirement of the exhaust and calibrating diesel engine control unit (ECU).
Therefore, it is desirable in this field for an air system control strategy that can satisfy the actual working condition of a diesel engine, and is relatively simple and easily implemented and calibrated.